Wednesday, March 4, 2009

When you get a blow-out

No I'm not talking about an economic one, or even one of your back tyre. I'm talking about a blow-out of the highlights kind.

What does this mean? Ever since the time of the dinosaurs (well at least before digital came along) we all took pictures on film.

Professionals used slide film and a technique called exposing for the highlights. What this term meant was that you had to calculate your exposure so that the brightest parts of the picture weren't so bright that they would be lost.

With film, too bright highlights would just turn into clear film - nothing recorded whatsoever. With digital it's pretty much the same - the histogram reaches Level 155 and anything beyond that means there are no details whatsoever.

So why should you, or even should you, care? Because sometimes you want to retain detail in those highlights. Take the image of snow men above. This was taken on my recent trip to the snow festival in Sapporo. As you can see they are all very, very white. Indeed I intended it that way.

But more importantly if you look at this histogram in the top right hand corner of the picture you can see that it is buffering up against the right hand edge, but not falling off it. In other words if you zoom into any of those pieces of snow you can actually see the texture in the snow.

If I had tried to make the snow look even more brilliantly white (by making the picture brighter) I would have ended up losing detail in the brightest parts of the picture, and when the picture is printed on paper you would just get bare paper - no ink being laid down - in those too bright areas. Not a good look.

So this is an example of where you want to keep an eye on your histogram while you're photographing. It's a good idea to set your camera so that the histogram appears after every image, that way you can keep an eye on it. If your picture is too bright (histogram too far to the right) or too dark (histogram too far to the left) you can use your exposure compensation button to fix it.

Of course that's assuming you want to keep the highlights. Many photography books tell you to keep the histogram within limits at all costs - but sometimes that's just not possible or desirable. Tomorrow I'll show you an example of when you can let highlights go.